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Full text of "Handbook Of Chemical Engineering - I"

PYROMETRY
411
discussed in this section which pertains more especially to high-temperature mercurial thermometers, and indicating and recording thermometers of the vapor pressure, liquid or gas filled and bimetallic types, a brief classification of which is given in the table following:
TABLE 2.—CLASSIFICATION OF THERMOMETERS General classification                                                               Subdivisions
Mercury in glass (mercurial)
Indicating and recording
Laboratory or chemical
Industrial Pressure
Bimetallic Graphite-metal expansion
Etched or engraved stem Enclosed scale (einschluss)
Vapor pressure Liquid filled Gas filled
400C
High-temperature Mercurial Thermometers.—In the laboratory the so-called engraved- or etched-stem thermometer is more extensively used in this country than the enclosed scale or einschluss type. Figure 1 shows characteristic types of etched-stem high-temperature ffl 53<f c laboratory thermometers. The first instrument (a) is the type used as a standard at the Bureau Standards in the range 300° to 530°C. The second thermometer (6) is a continuous-scale instrument used for making distillation tests and graduated 0° to 400°C. in 1° intervals for total immersion, the third instrument (c) is a 20 to 750°F. thermometer divided into 2° intervals for partial immersion and used in oil testing or other industrial work. In the use of high temperature thermometers, the ice-point graduation (0°C., 32°F.)
is desirable for checking purposes, since       ||                                            W 20
the indications of even the best made instruments fail to remain constant on repeated heating, due to a slow contraction of the glass in the bulb. Observations made from time to time of the change of readings in ice, may be used to correct the readings over the entire scale.
If it is necessary to use a thermometer for partial immersion, that is, with the bulb and only a part of the stem heated, it is advisable to use the continuous-scale type (6) or (c) rather than the standard type (a), especially if the enlargement in the capillary shown between the 0° and 295° marks projects out into the air which is much cooler than the bulb. Large and uncertain errors may be introduced by using a thermometer under these conditions.
295
FIG. 1.—Etched stem thermometers.